Field of the Invention
The invention relates to the mass spectrometric measurement of fragment ions with tandem time-of-flight mass spectrometers.
Description of the Related Art
Note: Instead of the statutory “unified atomic mass unit” (u), this document uses the “dalton” (Da), which was added in the last (eighth) edition of the document “The International System of Units (SI)” of the “Bureau International des Poids et Mesures” in 2006 on an equal footing with the atomic mass unit. As is noted there, this was done primarily in order to allow use of the units kilodalton, millidalton and similar.
Mass spectrometers can usually only determine the ratio of the ion mass to the charge of the ion. In the following, the term “mass of an ion” or “ion mass” can also refer to the ratio of the mass m to the number z of excess positive or negative elementary charges of the ion, i.e. the mass-to-elementary charge ratio (or mass-to-charge ratio, for short) m/z.
In the application document DE 10 2013 011 462.4 (C. Köster), which is to be included here by reference, time-of-flight mass spectrometers are described which have one or more Cassini reflectors instead of the usual Mamyrin reflectors. The prior art is also described in detail in this publication.
The term “fragment ion mass spectrum” or “daughter ion mass spectrum” usually refers to a mass spectrum of the fragment ions of a selected ionic species, while the ionic species selected for the fragmentation is usually called “parent ions”.
In time-of-flight mass spectrometers with ionization by matrix-assisted laser desorption (MALDI), a distinction is made between two types of fragmentation for the production of daughter ions—ISD fragmentation (“in-source decay”) and PSD fragmentation (“post-source decomposition”). To acquire daughter ion mass spectra by means of PSD, the energy of the laser pulses used for MALDI can be increased to the extent that, during the MALDI process, many metastable analyte ions are produced which decay into fragment ions (daughter ions) only after a first acceleration region, but before a reflector. It is also possible to produce unstable parent ions by means of collisions in a gas-filled collision chamber positioned between the first acceleration region and the reflector. In both cases, the parent ions for which a daughter ion mass spectrum is to be acquired have to be selected. The parent ions are usually selected using a parent ion selector which is positioned after the first acceleration region and before the reflector, and before the collision chamber, if there is one. If metastable parent ions have already decomposed between the first acceleration region and the parent ion selector, the fragment ions already formed here can also pass through the parent ion selector because they have essentially the same speed as the undecomposed parent ions, and therefore arrive at the parent ion selector at the same time as the undecomposed parent ions. The undecomposed parent ions and the daughter ions formed from the selected parent ions usually pass through a second acceleration region before they are separated in the reflector and measured as a daughter ion mass spectrum. Time of flight mass spectrometers and appropriate methods to acquire PSD daughter ion mass spectra are described in patent document DE 198 56 014 C2 (C. Köster et al., corresponding to GB 2 344 454 B and U.S. Pat. No. 6,300,627 B1), for example.
The acquisition of daughter ion mass spectra with ionization by matrix-assisted laser desorption (MALDI) consumes relatively large amounts of sample material. For PSD fragmentation, the energy of the laser pulses is greatly increased, thus at the same time increasing the sample consumption, in order to produce large numbers of metastable ions which are to decompose in a first straight flight path before the reflector. Furthermore, a separate daughter ion mass spectrum will also be acquired for each parent ion species selected. It is obviously a disadvantage that the restriction to one single parent ion species in each case means that many other ionic species are filtered out unused, thus causing relatively large amounts of sample to be consumed if several daughter ion mass spectra are to be measured. It is also disadvantageous that the acquisition of several daughter ion mass spectra has to be conducted sequentially, which requires longer measuring times.
An objective of the invention is to provide methods with which time-of-flight mass spectra of fragment ions are acquired quickly and with low sample consumption, in particular by ionization with matrix-assisted laser desorption (MALDI).